NORTHERN PIKE (Esox lucius) CONTROL in the MIDDLE ......NORTHERN PIKE (Esox lucius) CONTROL in the...
Transcript of NORTHERN PIKE (Esox lucius) CONTROL in the MIDDLE ......NORTHERN PIKE (Esox lucius) CONTROL in the...
-
NORTHERN PIKE (Esox lucius) CONTROL in the
MIDDLE GREEN RIVER, UTAH 2001-2006
L. Monroe and T. Hedrick Utah Division of Wildlife Resources
152 East 100 North Vernal, UT 84078
SYNTHESIS REPORT February 22, 2008
Prepared for:
Upper Colorado River Basin
Endangered Fish Recovery Program Project Number: 109
Publication Number 08-XX
Utah Division of Wildlife Resources 1592 West North Temple Salt Lake City, UT 84114
-
2
ACKNOWLEDGEMENTS AND DISCLAIMER
This study was funded by the Recovery Implementation Program for Endangered Fish
Species in the Upper Colorado River Basin. The Recovery Program is a joint effort of the U.S.
Fish and Wildlife Service, U.S. Bureau of Reclamation, Western Area Power Administration,
states of Colorado, Utah, and Wyoming, Upper Basin water users, environmental organizations,
the Colorado River Energy Distributors Association, and the National Park Service.
The authors would like to acknowledge the work of the previous principal investigators
on this project: Ron Brunson and Kevin Christopherson, and thank Brent Sheffer and Ben
Williams for their work on the project from 2001-2005. We would also like to thank the
reviewers for their comments and critiques.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use by the authors, the Fish and Wildlife Service, U.S. Department of
Interior, or members of the Recovery Implementation Program.
-
3
TABLE OF CONTENTS
ACKNOWLEDGEMENTS AND DISCLAIMER..................................................................... 2
TABLE OF CONTENTS ............................................................................................................. 3
LIST OF TABLES ........................................................................................................................ 5
LIST OF FIGURES ...................................................................................................................... 6
LIST OF KEY WORDS ............................................................................................................... 7
EXECUTIVE SUMMARY .......................................................................................................... 7
INTRODUCTION......................................................................................................................... 9
GOALS AND OBJECTIVES .................................................................................................... 11
STUDY AREA............................................................................................................................. 12
METHODS .................................................................................................................................. 13 SAMPLING.................................................................................................................................. 13 CATCH PER UNIT EFFORT .......................................................................................................... 14 AGE ANALYSIS .......................................................................................................................... 15 STOMACH CONTENT ANALYSIS ................................................................................................. 15
RESULTS .................................................................................................................................... 16 SAMPLING.................................................................................................................................. 16
2001....................................................................................................................................... 16 2002....................................................................................................................................... 16 2003....................................................................................................................................... 17 2004....................................................................................................................................... 17 2005....................................................................................................................................... 18 2006....................................................................................................................................... 18
CATCH PER UNIT EFFORT .......................................................................................................... 19 AGE ANALYSIS .......................................................................................................................... 20 STOMACH CONTENT ANALYSIS ................................................................................................. 21
DISCUSSION .............................................................................................................................. 22 CATCH PER UNIT EFFORT .......................................................................................................... 22 FACTORS AFFECTING CATCH RATES.......................................................................................... 22
Flows..................................................................................................................................... 23 Net placement........................................................................................................................ 23 Equipment issues................................................................................................................... 24 Crew turnover ....................................................................................................................... 24
CATCH PER UNIT EFFORT SUMMARY ........................................................................................ 24 AGE ANALYSIS ........................................................................................................................ 266 STOMACH CONTENT ANALYSIS ............................................................................................... 288
CONCLUSIONS ......................................................................................................................... 29
-
4
RECOMMENDATIONS............................................................................................................ 31
LITERATURE CITED .............................................................................................................. 32
-
5
LIST OF TABLES
TABLE 1. CATCH RATES AND TOTAL NUMBER OF NORTHERN PIKE REMOVED DURING THE NORTHERN PIKE REMOVAL PROJECT (ONLY) FROM THE MIDDLE GREEN RIVER: 2001 – 2006. ..................................................................................................... 34
TABLE 2. EFFORT EXPENDED BY GEAR TYPE DURING THE NORTHERN PIKE REMOVAL PROJECT (ONLY): 2001-2006............................................................................... 34
TABLE 3. EFFORT AND NUMBER OF NORTHERN PIKE REMOVED DURING OTHER PROJECTS IN THE MIDDLE GREEN RIVER: 2001-2006. ..................................................... 34
TABLE 4. NORTHERN PIKE CAPTURES BY LOCATION: 2001-2006. .............................. 35
TABLE 5. NORTHERN PIKE CATCH RATES FROM THE UPPER AND MIDDLE YAMPA RIVER REMOVAL PROJECTS.................................................................................................. 35
TABLE 6. AGE GROUPINGS BY YEAR: 2001-2006. ............................................................. 35
TABLE 7. INFORMATION COLLECTED ON THE FOUR NORTHERN PIKE ORIGINALLY TAGGED IN THE YAMPA RIVER AND SUBSEQUENTLY REMOVED FROM THE GREEN RIVER. ...................................................................................................... 35
-
6
LIST OF FIGURES
FIGURE 1. THE UPPER COLORADO RIVER BASIN WITH THE NORTHERN PIKE CONTROL REACH HIGHLIGHTED......................................................................................... 36
FIGURE 2. LENGTH FREQUENCY OF NORTHERN PIKE CAUGHT IN THE MIDDLE GREEN RIVER: 2001. ................................................................................................................ 37
FIGURE 3. LENGTH FREQUENCY OF NORTHERN PIKE CAUGHT IN THE MIDDLE GREEN RIVER: 2002-2003......................................................................................................... 37
FIGURE 4. LENGTH FREQUENCY OF NORTHERN PIKE CAUGHT IN THE MIDDLE GREEN RIVER: 2004-2006......................................................................................................... 38
FIGURE 5. AVERAGE AND PEAK ANNUAL FLOW (CUBIC METERS PER SECOND) AS MEASURED AT THE GREEN RIVER GAUGE AT JENSEN, UT, COMPARED WITH THE NORTHERN PIKE CATCH-PER-UNIT-EFFORT (NUMBER OF NORTHERN PIKE/HOUR OR NUMBER OF NORTHERN PIKE/FYKE-NET NIGHT) IN THE MIDDLE GREEN RIVER FROM 2001 – 2006 (NORTHERN PIKE REMOVAL PROJECT ONLY)..... 38
FIGURE 6. AVERAGE AND PEAK ANNUAL FLOW (CUBIC METERS PER SECOND) AS MEASURED AT THE GREEN RIVER GAUGE AT JENSEN, UT, COMPARED WITH THE NORTHERN PIKE CATCH-PER-UNIT-EFFORT (NUMBER OF NORTHERN PIKE/HOUR ) IN THE MIDDLE GREEN RIVER FROM 2001 – 2006 (ALL OTHER PROJECTS). ................................................................................................................................. 39
FIGURE 7. NORTHERN PIKE AGE ESTIMATES BY YEAR................................................ 39
FIGURE 8. NORTHERN PIKE AGE CLASS BY YEAR. ........................................................ 40
FIGURE 9. AVERAGE AND PEAK ANNUAL FLOW (CUBIC METERS PER SECOND) AS MEASURED AT THE GREEN RIVER GAUGE AT JENSEN, UT, COMPARED WITH THE NUMBER OF NORTHERN PIKE OBSERVED (PRE-2001) OR REMOVED (2001 AND LATER) IN THE MIDDLE GREEN RIVER FROM: 1996 – 2006. ........................................... 40
-
7
LIST OF KEY WORDS
Northern pike, Esox lucius, nonnative removal, cleithra, nonnative fish, middle Green River
EXECUTIVE SUMMARY
Beginning in 2001, researchers with the Utah Division of Wildlife Resources (Division)
in Vernal, UT, began an intensive northern pike (Esox lucius) removal program in razorback
sucker (Xyrauchen texanus) critical habitat of the middle Green River. Captures of northern pike
during the Division’s basin-wide surveys had increased from 1997 to 2000 and participants in the
Upper Colorado River Endangered Fish Recovery Program (Program) were concerned that large
numbers of pike would threaten recovery of native, endangered fishes in the Green River.
During this effort, crews were asked to capture and remove all northern pike encountered
and to develop a northern pike control program or methodology, which could subsequently be
evaluated for its effectiveness. They did this by utilizing multiple gear types: electrofishing, fyke
netting, and trammel netting. Catch rates were highest for all gear types in 2001 and although
catch rates varied over the course of this study, the overall trend was a gradual reduction (in both
catch rates and numbers removed) from 2001 to 2006. Catch rates in this reach were low
compared with the Yampa River.
From 2001 to 2005, researchers collected cleithra from northern pike and counted annuli
to determine ages. Results of this analysis were grouped into young-of-year, juvenile, and adult
age classes. Northern pike removed from the middle Green River from 2001 to 2003 were
predominantly adult fish. In 2004, most fish were juvenile fish, and in 2005, pike were evenly
distributed between juvenile and adult. Only three pike were identified as young-of-year over the
Formatted
-
8
study period. In comparison, researchers in the Yampa River see all size and age classes of pike,
including young-of-year.
Researchers examined the stomach contents of all northern pike collected from 2002 to
2005. The overwhelming majority of stomachs were empty; only 13 of 144 pike stomachs
contained fish remains. Of these, four pike had consumed native fish: three recently stocked
razorback suckers and one bluehead sucker (Catostomus discobolus: total length (TL) 250mm).
Lengths of pike that had consumed native fish ranged from 597 mm to 834 mm.
Based on decreasing catch rates and limited reproduction in the study area we were able
to effectively reduce numbers of northern pike in the middle Green River. Available northern
pike habitat within the study area appears to be limited and is easily sampled. We recommend a
continuation of specific efforts to remove northern pike in the early spring. This coupled with
opportunistic removal of northern pike throughout the year during other Program projects should
effectively reduce their threat to the recovery of endangered fish in the middle Green River.
-
9
INTRODUCTION
Nonnative fishes have become established in rivers of the upper Colorado River basin,
with certain species implicated in the reduction in distribution and abundance of native fishes,
primarily through predation and competition (Hawkins and Nesler 1991; Lentsch et al. 1996;
Tyus and Saunders 1996). The Upper Colorado River Endangered Fish Recovery Program
(Program) has determined that control of nonnative fish in the upper Colorado River basin is
essential to the recovery of the four endangered fish species: Colorado pikeminnow
(Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha), and
bonytail (Gila elegans). Experts in the Upper Colorado River Basin have rated northern pike
(Esox lucius) as one of the six nonnative species of greatest concern (Hawkins and Nesler 1991).
It is thought that northern pike were originally introduced as a game fish in Elkhead
Reservoir, a small reservoir on Elkhead Creek (a tributary to the Yampa River in northern
Colorado), in 1977 (Figure 1). The subsequent invasion of the Yampa River was likely due to
this original introduction into Elkhead Reservoir (Tyus and Beard 1990). However, Hawkins et
al. (2005) hypothesize that Elkhead Reservoir was not the original source of northern pike in the
Yampa River due to discrepancies in stocking records from this period. Regardless, since the
original invasion, northern pike have established a reproducing population in the upper Yampa
River and have expanded their numbers and range throughout this river (Tyus and Beard 1990).
Pike were not found in the Green River until after this initial invasion suggesting that the species
moved downstream from the Yampa River into critical habitat of the Green River (Tyus and
Beard 1990; Hawkins et al. 2005). Here, as in the Yampa River, they pose a competitive and
predatory threat to the endangered fishes. Since 2001, Division of Wildlife Resources (Division)
data show that four tagged northern pike ranging from 720 mm to 826 mm have moved from the
-
10
Yampa River downstream into the middle Green River, into critical habitat for razorback sucker.
These fish moved down from between Yampa River Kilometer (RKM) 119.7 – 242.9 (River
Mile (RM) 74.4 - 150.9) for total movements between 192.1 and 306.5 river kilometers (119.4
and 190.5 river miles).
Capture rates of northern pike in the Green River in the late 1990’s indicated a rapid
increase in the adult population: the number of northern pike captured during basin-wide
monitoring increased from 48 collected in 1997 to 202 in 1999. In addition, in 2002, U.S. Fish
and Wildlife Service crews observed numerous young-of-year (YOY) northern pike in Old
Charley Wash on the Ouray National Wildlife Refuge (Green River RKM 401.0 (RM 249.2)) (K.
Christopherson, pers. comm.) (Figure 1). This was dealt with through the use of Rotenone, a
piscicide; however, the possibility of an established northern pike population was still quite
alarming. Northern pike tend to prefer soft-rayed food fishes to pan fish or bullheads (Beyerle
and Williams 1968) and because of the potential predatory threat to native, endangered species,
the Program became very concerned and initiated a northern pike removal program in 2001. The
Program was also concerned that northern pike would pose a predatory threat to all native fishes,
including the non-endangered, “at risk” species such as roundtail chub (Gila robusta),
flannelmouth sucker (Catostomus latipinnis) and bluehead sucker (Catostomus discobolus).
-
11
GOALS AND OBJECTIVES
At the beginning of this study, in 2001, crews were asked to develop an effective
northern pike control program from Split Mountain in Dinosaur National Monument to Sand
Wash (the study area will be discussed in further detail in the next section), focusing their
removal efforts in areas of known northern pike concentrations. The goal of this control program
was to sufficiently reduce the abundance of adults such that predatory and competitive impacts
on growth, recruitment, and survival of endangered and other native fishes were minimized.
The study objectives were to:
1. Capture and remove (lethal) adult northern pike from reaches of the middle Green
River.
2. Reduce the abundance of adult northern pike in the middle Green River. After
2003, this objective was changed to “Maintain low occurrence of adult northern
pike in the middle Green River.”
3. Determine the efficacy of removal efforts.
4. Identify the means and levels of northern pike control necessary to minimize the
threat of predation/competition on endangered and other native fishes.
The end product included herein is an evaluation of the effectiveness of this northern pike
removal program.
-
12
STUDY AREA
The Utah Division of Wildlife Resources (Division) Vernal office was tasked with
conducting northern pike removal efforts in critical habitat of the middle Green River. This
includes the Green River from the Split Mountain boat ramp in Dinosaur National Monument
(RKM 513; RM 319) to the Sandwash boat ramp (RKM 346; RM 215), which is the staging
location for rafting trips through Desolation Canyon (Figure 1). This reach and removal
localities therein were selected based on northern pike observations from previous sampling
activities. Known concentration areas for northern pike in this reach include the mouths of Brush
Creek (RKM 490; RM 304.5), Cliff Creek (RKM 487; RM 302.9), Stewart Lake Drain (RKM
483; RM 300.0), Ashley Creek (RKM 481; RM 299.0) and Sportsman Drain (RKM 477; RM
296.6) (Figure 1). Each of these areas is characterized by slow water and submerged vegetation,
similar to wetland habitat, which is ideal for northern pike and their spawning activities (Becker
1983; Bry 1996).
Higher gradients and an abundance of cobble, rubble, and gravel substrates characterize
the uppermost section of the reach within Dinosaur National Monument. The lower section of
this reach down to Sandwash boat ramp is a much lower gradient with primarily silt and/or sand
substrates. Two major tributaries, the Duchesne and the White rivers flow into the Green River
within this section.
-
13
METHODS
Sampling
In all years, sampling began in March, shortly after ice-off and continued through spring
as river flows began to increase. This is the general time period when northern pike become
active and move to shallow water areas including wetlands and flooded tributaries (Sigler and
Sigler 1996). Selected reaches were sampled two to three times weekly. Crews mainly used three
different methods for removal of northern pike: electrofishing, trammel netting, and fyke netting.
Electrofishing was performed using 4.88 m aluminum welded jon boats equipped with 5.0
Smith-Root GPP electrofishers. This was done in locations such as Stewart Lake Drain, the
mouth of Ashley Creek, and larger backwaters. Shoreline electrofishing was also effective for
northern pike removal during other projects such as the Colorado pikeminnow abundance
estimates, another spring project. Trammel nets were set at the mouth of tributaries and large
backwaters to trap pike that might use these areas as refuge to escape increasing spring flows.
Trammel nets were used mainly in conjunction with electrofishing to increase catch rates. Fyke
nets were also set in tributary mouths and backwaters to trap fish as they went into these areas.
Sampling crews conducted removal activities in a manner that minimized potential
negative impacts to endangered fish. For instance, sampling was curtailed or re-directed when
Colorado pikeminnow were staging in tributary mouths or backwater habitats prior to spawning;
when razorback sucker were congregated on and near the spawning bars; and following recent
releases of hatchery reared endangered fish.
In 2001, gear was placed in a number of locations from Split Mountain to the Ouray
Bridge (RKM 399; RM 248). By 2002, northern pike removal locations had become condensed
mainly to the area from Split Mountain to Ashley Creek, though crews were still somewhat
-
14
successful at sampling lower in the sampling reach near Wyasket Bottom (RKM 410; RM 255)
and the White River (RKM 396; RM 246). By 2004, however, the main concentration areas were
Brush Creek, Cliff Creek, Stewart Lake inlet and drain, Ashley Creek, and the general Jensen
area (Figure 1).
All northern pike captured from all middle Green River sampling projects were counted,
weighed (g) and measured (mm) for total length (TL). All nonnative fish were euthanized and
removed; native fish were released at the site of capture. Endangered fish species were scanned
for a PIT (passive integrated transponder) tag, tagged if needed, and then released near the area
of capture. Nonnative removal and evaluation efforts, which included tagging and marking
native, endangered and target nonnative fishes, were also being conducted by other researchers
and agencies in other areas of the middle Green River and Yampa River. Therefore, sampling
crews examined all native, endangered and target nonnative fish for tags or marks and recorded
pertinent information. This information was then reported to principal investigators as
appropriate and included in annual reporting.
Catch Per Unit Effort
Catch-per-unit-effort (CPUE) was either calculated as fish/hour (electrofishing and
trammel netting) or fish/fyke-net night (fyke-netting) (one fyke net set out for one night and
usually left for a 24-hour period is equal to one fyke-net night). Fyke nets set for shorter periods
of time were recorded in tenths of a fyke-net night for purposes of calculating CPUE. No
statistical analyses were performed on CPUE over the study period because removal efforts were
not done in a statistically consistent manner (removal efforts were not completed by pass, but
rather by distinct sampling occasions). Catch rates from the middle Green River were compared
-
15
between years and to catch rates from the Yampa River, the likely source population, to discuss
the effectiveness of the program.
Age Analysis
Cleithra were used to age northern pike (n=320) collected from 2001 through 2005.
Cleithra were first cleansed in boiling water to remove soft tissue. Following accepted protocols
(Campana 2001), crewmembers counted annuli under a microscope to determine age. To better
make sense of the cleithra data, age results were lumped into three groups: young-of-year (YOY:
300 mm or less), juvenile (ages 1-3: 301-520 mm), and adult (4+: 521+mm). This length at age
grouping was consistent with Brown (1971) who aged northern pike in streams in Montana, and
Scott and Crossman (1973) who found that both male and female northern pike in the northern
United States will be mature by age four (males will mature at two to three years and females at
three to four, but all will be mature at age four).
Stomach Content Analysis
Stomach content analysis was conducted from 2002 through 2005. In the field, stomachs
were individually preserved in 95% alcohol. In the laboratory, contents were identified to
species, if possible. Lengths were taken on all native fish identified within the stomach. Crews
did not attempt to identify partially digested materials.
Formatted
-
16
RESULTS
Sampling
2001 Electrofishing for northern pike removal began on 19 March 2001, ended on 08 May, and
lasted 34 sampling days (Table 1). Crews fished for a total of 19.1 hours for this project (Table
2). The fyke netting effort began on 20 March and was completed on 01 June. This effort
consisted of 421 fyke net nights. Crews also spent 37.7 hours electrofishing with a trammel net
using the “block and shock” method where an area is closed off using a trammel net and fish are
then chased into the trammel net as they try to escape the electrofishing equipment. This effort
began on 20 March and ended on 29 June. Between the three efforts, 222 northern pike were
removed from the Green River (Table 2). Crews also electrofished the middle Green River for
the Colorado pikeminnow abundance estimates this year. This effort took 253 hours and
removed an additional 29 pike from the reach for a grand total of 251 (Table 3). Mean total
length of all pike captured was 613 mm (170 mm – 871 mm) (Table 1; Figure 2). Northern pike
were predominantly removed between middle Green River RM 303 (Thunder Ranch) and RM
294 (Walker Hollow), though crews removed a few pike from the Split Mountain reach and
around the town of Ouray and the Duchesne River (Table 4).
2002 Electrofishing for northern pike began on 02 April 2002, ended on 10 June, and took 20
sampling days to complete (Table 1). Crews fished for a total of 28.1 hours for this project
(Table 2). Fyke netting began on 10 April and ended on 23 May. This effort consisted of 165
fyke net nights. Crews also spent 5.3 hours trammel netting between 16 April and 10 June.
-
17
Between all three efforts, a total of 32 northern pike were removed (Table 2). The Colorado
pikeminnow abundance estimates continued in 2002 and crews removed an additional 10
northern pike during this effort (182 hours) (Table 3). Mean total length of all pike removed was
633 mm (295 mm – 815 mm) (Table 1; Figure 3). Concentration areas during 2002 remained the
Thunder Ranch to Walker Hollow reach, though quite a few were still captured near the town of
Ouray and the Duchesne River (Table 4).
2003 Electrofishing for northern pike began on 31 March 2003, ended on 27 June, and took 15
sampling days to complete (Table 1). Crews fished for a total of 65.5 hours for this project
(Table 2). Fyke netting began on 12 March and ended on 20 June. This effort consisted of 103
fyke net nights. No trammel netting occurred this year for northern pike removal. Between the
two efforts, a total of 7 northern pike were removed (Table 2). The Colorado pikeminnow
abundance estimates continued in 2003 and crews removed an additional 16 northern pike during
this effort (190 hours) (Table 3). Mean total length of all northern pike was 668 mm (423 mm –
984 mm) (Figure 3). Most pike were again removed from the Thunder Ranch to Walker Hollow
reach and the Split Mountain reach (Table 4).
2004 Electrofishing began on 24 March 2004, ended on 14 June, and took 14 sampling days to
complete (Table 1). Crews fished for a total of 14.7 hours for this project (Table 2). Fyke netting
began on 22 March and ended on 14 June. This effort consisted of 93 fyke net nights. Crews
returned to trammel netting between 23 April and 13 May and spent a total of 2.4 hours on this
-
18
activity in 2004. Between the three efforts, 18 northern pike were removed (Table 2). The
Colorado pikeminnow abundance estimates did not occur in 2004; however, crews spent 147
hours electrofishing for another project, smallmouth bass removal, and removed an additional
nine northern pike from the river (Table 3). Mean total length of all pike removed was 558 mm
(371 mm – 834 mm) (Table 1; Figure 4). The greatest concentration of pike was again removed
from the Thunder Ranch to Walker Hollow reach in 2004, though seven pike were removed
between Razorback Bar (Green River RKM 500.5 (RM 311)) and Thunder Ranch (Green River
RKM 487.6 (RM 303) as well (Table 4).
2005 Electrofishing for northern pike removal began on 11 April 2005, ended on 25 June, and
took 32 sampling days to complete (Table 1). Crews fished for a total of 8.5 hours for this project
(Table 2). Fyke netting began on 21 March and was completed on 16 June. This effort consisted
of 227 fyke net nights. Crews continued to trammel net for northern pike between 24 March and
24 June and spent a total of 26 hours on this activity in 2005. Between the three efforts, 29
northern pike were removed (Table 2). Crews spent 80.4 hours electrofishing for smallmouth
bass removal and removed an additional three northern pike from the river (Table 3). Mean total
length of northern pike was 638 mm (388 mm – 870 mm) (Table 1; Figure 4). By far, the
majority of pike were again removed from the Thunder Ranch to Walker Hollow reach of the
middle Green River (Table 4).
2006
-
19
Electrofishing for northern pike began on 11 May 2006, ended on 12 May, and only
lasted one sampling day (most electrofishing effort in 2006 was done during other projects)
(Table 1). Crews fished for a total of 3.3 hours for this project (Table 2). Fyke netting began on
27 March and was completed on 31 May. This effort consisted of 52 fyke net nights. Crews
continued to trammel net for northern pike between 27 March and 30 March and spent a total of
two hours on this activity in 2006. Between the three efforts, a total of three northern pike were
removed (Table 2). Crews returned to the Colorado pikeminnow abundance estimates (190.4
hours) and continued smallmouth bass removal (152.2 hours) and removed an additional 17
northern pike during these projects (Table 3). Mean total length of all pike removed was 549 mm
(365 mm – 790 mm) (Table 1; Figure 4). Most pike were removed from the Split Mountain reach
in 2006 (Table 4).
Catch Per Unit Effort
A total of 414 northern pike was removed over the six years of this removal effort. Total
catch per effort over the six removal years by gear type was as follows: 0.60 fish/hour
electrofishing, 0.16 fish/fyke-net night using fyke nets, and 0.76 fish/hour trammel netting and
electrofishing. In general, following the removal of 251 northern pike and the highest catch rates
in 2001, both numbers of pike removed and catch-per-effort have gradually declined and
remained low through 2006 (Table 1; Figures 5 and 6).
This is also true for northern pike captured during other studies. Catch rates of pike
during the Colorado pikeminnow abundance estimates (2001-2003; 2006) and smallmouth bass
removal (2004-2006) started low (0.11 pike/hour and 0.06 pike/hour for each project
respectively) and have remained low (0.08 pike/hour and 0.02 pike/hour in 2006 for each project
-
20
respectively) (Table 3). Catch rates over the entire period of pikeminnow abundance estimates
were 0.08 fish/hour and 0.03 fish/hour for smallmouth bass removal.
No statistical analyses were performed on the data from the northern pike removal project
because pike were not removed by pass. We can, however, qualitatively compare our catch rates
with those from the Yampa River, a likely source of middle Green River pike. Reviewing annual
reports and preliminary data from the northern pike removal projects there, catch rates are
slightly higher, but have also been declining since the beginning of the projects. In the upper
Yampa River, between Hayden and Craig, CO (RKM 275 – RKM 215.7 (RM 170.9 – RM 134)),
electrofishing catch rates have gone from 8.0 pike/hour in 2004 down to 4.3 pike/hour in 2006
(Table 5). Electrofishing catch rates in the middle Yampa River (RKM 223.7 - RKM 95.0 (RM
139 – RM 59)) are also higher than in the middle Green River and have declined over the study
period: 2.15 pike/hour in 2004 down to 1.43 pike/hour in 2006. Middle Yampa River removal
crews did not set fyke nets until 2006. Catch rates for this gear type were low (0.12 fish/fyke-net
night), which is similar to those seen in the middle Green River.
Age Analysis
In 2001, cleithra from 217 northern pike indicate age classes from young-of-year (YOY)
through age 10 were present. Most pike collected were two to four years old and ranged from
400 to 800 mm in length. Only 12 pike were aged older than six years (Figure 7). All but one fish
(a 295 mm YOY) captured in 2002 were identified as two to seven years old (Figure 7). The
majority of these were actually in the four to five year-old range. In 2003, again, most fish were
identified as four or five years old. Only three fish were identified as one- or two-year-old fish.
The remaining fish were age-six or older. In 2004, the majority of fish were one-year-old fish,
Deleted: ¶
-
21
but the remaining fish seemed evenly distributed from age-two to age-seven. Finally, in 2005,
fish were relatively evenly spread between age-one and age seven.
As explained earlier, fish were lumped into YOY, juvenile, and adult fish due to a great
deal of overlap in the age-at-length estimates. Over the course of the study, only three young-of-
year (YOY) fish were captured; this was in 2001 and 2002. These fish were 170, 295, and 390
mm. The 390 mm fish was captured early in 2000 and was likely an age-1 fish with no distinct
cleithra markings. In 2002 and 2003, most fish were adult fish and ranged from 520 to 984 mm.
In 2004, this shifted to most fish being predominantly juvenile fish, ranging in length from 371
to 480 mm. In 2005, pike numbers were low and juvenile/adult fish were evenly represented in
the catch (Figure 8). Length ranges for all aged fish from 2001 to 2005 are included in Table 6.
Stomach Content Analysis
Over the course of the study, most extracted stomachs were empty. Only 13 of 144 fish
had stomach contents recorded. Of these 13, five had minnows, shiners, and sunfish listed as
their stomach contents. Four additional northern pike had potentially larger prey items in the
stomachs (though prey size was rarely recorded for nonnative fish): channel catfish, black
bullhead, white sucker, and one 178 mm smallmouth bass (this pike was 620 mm total length).
Four northern pike had consumed native fish: three of which were hatchery reared razorback
suckers (recently stocked in the Green River); the fourth native fish was a bluehead sucker. All
native fish were near 250 mm in size and were eaten by northern pike ranging from 597 mm to
834 mm.
-
22
DISCUSSION
Catch Per Unit Effort
During the six years of the removal effort for northern pike, electrofishing and trammel
netting catch rates decreased; fyke netting catch rates have varied, but essentially decreased over
the years (Table 1). This is true also for northern pike catch rates during other projects in the
middle Green River, such as the Colorado pikeminnow abundance estimates and smallmouth
bass removal (Table 3). Among all gear types, electrofishing was the most efficient gear type
throughout the entire study, although trammel netting was efficient at capturing northern pike in
most years when used in conjunction with electrofishing (Table 1).
In comparison with catch rates in parts of the basin with established pike populations
(i.e., the Yampa River), catch rates in the middle Green River are low and effort is considered to
be at a preventative level (i.e., with minimal effort we are removing a similar number of northern
pike each year and preventing the establishment of a reproducing northern pike population).
Specific efforts to control northern pike in the middle Green River have decreased from 2001
(19.1 electrofishing hours; 421 fyke net nights; and 37.7 trammel netting hours) to 2006 (3.3
electrofishing hours; 52 fyke net nights; and 2 trammel netting hours).
Factors Affecting Catch Rates
Because this was a six-year study period, many things changed over the years that may
have influenced these catch rates (besides a decrease in the number of northern pike in this
reach): flows, net placement, equipment issues, and crew turnover.
-
23
Flows Peak and average flows varied over the study period. Again, we did not do any statistical
analyses on these data; however, by overlaying peak and average annual flows onto catch rates
for northern pike in the middle Green River, some patterns do emerge (Figure 5). For the
northern pike removal project, it initially appears that low flow may have been responsible for
the decrease in catch rates between 2001 and 2002 for both electrofishing and fyke netting. It is
difficult to conclude, however, that the decrease in catch rates were entirely a result of low flows
because catch rates did not rebound with an increase in flows in 2003 or any subsequent year.
Over the same low-flow period, however, there was no real decrease in trammel netting catch
rates, suggesting that trammel netting may be a more consistent method of removal over varying
hydrologies.
Flows may affect catch rates of northern pike during the Colorado pikeminnow
abundance estimates (Figure 6), during which northern pike catch rates mirrored patterns in
annual peak flow. Northern pike catch rates during smallmouth bass removal seem to show the
opposite trend (Figure 6). It is important to realize, however, that these two projects are
conducted at very different times of the year in terms of hydrology. The pikeminnow abundance
estimate is completed in the spring before peak flows, whereas smallmouth bass removal is done
after peak flows recede. In 2005 and 2006, most of the smallmouth bass removal was actually
done during base flows. Therefore, the number of northern pike captured during these studies
may instead be a result of the timing of the projects, not necessary flows.
Net placement Specific net set locations varied little between 2001 and 2005, and only to improve catch
rates. For example, if crews had set a net in the mouth of Ashley Creek that was not fishing well
after one or two days, they moved it within this same general location to better catch fish as they
-
24
moved into or out of the creek. With a large scale turnover in personnel, net placement changed
the most between 2005 and 2006, which may have affected catch rates. This potential
complicating factor is discussed in the final paragraph of this subsection.
Equipment issues River otters (Lutra canadensis) have been known to chew holes in nets and eat the
captured fish. In addition, fyke nets can be washed away when flows rise quickly. From 2001 to
2005, complications associated with otters and river flow were considered infrequent and of little
consequence. In 2006, river otters did compromise fyke-netting operations. Crews responded by
shifting efforts from fyke-netting to more electrofishing and trammel netting. If this were a major
factor influencing catch rates, however, one would predict a decrease in fyke-netting catch rates
and a subsequent increase in catch rates for the other two gear types as effort is redistributed;
however, this did not happen (Table 1).
Crew turnover While project personnel remained consistent from 2001-2005; the entire crew was
replaced in 2006. Changes in personnel in 2006 resulted in some minor changes in specific net
set locations and a general decrease in overall sampling effort as the crew was not in place until
late spring. This factor does not, however, explain the observed decrease in catch rates over the
entire study period.
Catch Per Unit Effort Summary
-
25
It is not likely that the previous factors alone or even combined, are responsible for the
observed reduction in northern pike catch rates. As mentioned, the two latter issues: equipment
issues and crew turnover likely only affected catch rates (if at all) between pre-2006 and 2006. It
does not explain the trend in decreasing catch rates between 2001 and 2005. Essentially, flows
may have affected catch rates to a certain extent; however, from Figure 9, it appears that while
spring peak flows decreased from 1997 to 2002, the numbers of northern pike increased (we do
not have catch rates before 2001). In addition, regardless of what happened with flows between
2001 and 2006, numbers of pike essentially decreased or remained stable during this period. This
essentially suggests that the removal effort has been the number one factor affecting catch rates
in the middle Green River.
This project began in 2001 with the removal of 222 northern pike (2.77 fish/hour
electrofishing; 0.31 fish/fyke net night; 1.03 fish/hour trammel netting) and an additional 29 pike
captured during the Colorado pikeminnow abundance estimates (0.11 fish/hour). Based on catch
rate data and absolute numbers of pike removed in subsequent years, the population never
rebounded after this initial effort. Although we cannot say for certain why this species has not
proliferated in this river reach as other nonnative species have over the same time period (i.e.,
smallmouth bass), it is likely because this reach of river is not a source of northern pike in the
basin (i.e., pike have only limited capacity to spawn in this reach of river and we have thus far
been effective at further limiting their spawning capability through this study). Other researchers
in the Yampa River basin see higher catch rates of northern pike (Table 5). If the Yampa River is
the source of the middle Green River population, reductions in catch rates, the lack of YOY fish
in most years, and the observed size shifts between years can all be explained.
-
26
Age Analysis
Age results, even grouped into YOY, juvenile, and adult age classes, show an interesting
shift from predominantly adults in 2001, 2002, and 2003 to predominantly juveniles in 2004 and
an even number of both in 2005 (Figure 8). The inability to follow an age class through time
suggests that in 2003 and 2004, removal efforts disturbed the population to the extent that most
individuals are now removed each year and new individuals from other parts of the basin replace
them. We do have definitive evidence of four individuals moving down from the Yampa River
into critical habitat for razorback sucker in the middle Green River over the study period (Table
7). These fish moved between 192.1 and 306.5 river kilometers (119.4 and 190.5 river miles)
downstream to get to the middle Green River near Jensen. We therefore know that downstream
movement occurs; we can only assume it occurs to the extent that the majority of pike in the
middle Green River were spawned in the Yampa River.
Beginning in 2006, northern pike captured in the middle Green River were sent to
researchers at Colorado State University for analysis of stable isotopic signatures. If pike sources
in the Yampa River drainage have a strong isotopic signature such that this signature imprints
itself on the resident fish, we will eventually know the source of middle Green River pike.
The cleithra results suggest that northern pike may have successfully spawned in 2001
and 2002; however, because numbers of these younger fish in the river remained low (we
detected only one YOY in both 2001 and 2002), it is more likely that these fish migrated down
from the Yampa River, thus allowing us to keep this removal effort at a preventative level.
There are some issues to consider with the cleithra results. The researchers at the time
certainly believed they were accurate in their age determinations; however, Quist et al. (2006)
point to difficulties with aging catostomids of the upper Colorado River basin based on cleithra
-
27
analysis. Quist et al. (2006) evaluated between-reader precision of aging results for various
catostomid species from analysis of fin rays, cleithra, opercular bones, and scales in comparison
with age results from otolith analysis. They found that one reader was nearly accurate at
identifying otolith age using the cleithra, but when this reader did err, he generally
underestimated otolith age. The second reader consistently underestimated otolith age by two to
three years, suggesting that it is quite easy to underestimate the age of catostomid species using
cleithra aging techniques. In addition, Sharp and Bernard (1988) also point to difficulty in aging
lake trout using cleithra techniques. A direct quote taken from their abstract exemplifies their
concerns with using cleithra for aging lake trout in Alaska lakes, “Estimated ages from cleithra
and whole vertebrae were, respectively, too imprecise and too low for these structures to be
useful in age validation studies.”
Even though these studies looked at accuracy of age analysis using cleithra from other
species, their findings suggest that the use of cleithra in aging fish is not extremely accurate.
Given these findings and the significant overlap of lengths at various ages (i.e., in 2001, a 980
mm fish was identified as an age-7 fish, but a 559 mm fish was identified as age-9), we felt more
comfortable grouping pike into YOY, juvenile, and adult. It is possible, according to data from
CDOW (unpublished) that the 980 mm fish could have come from Stagecoach Reservoir; this is
within the observed range of normal growth for northern pike in this reservoir. However, the 559
mm fish shows extremely slow growth rates for an age-9 fish based on observations of
Catamount and Stagecoach reservoirs and the Yampa River. Since we cannot be certain whether
the observed variation is due to error in the aging results or to variation in growth rates, this age
class grouping likely reduced much of that error (i.e., both of the fish mentioned above are adult
-
28
fish, thus removing any error that might exist between whether these are truly age-7 and age-9
fish).
Stomach Content Analysis
The stomach content analysis revealed very little new information. Clearly, northern pike
eat native fish and can eat native fish that have recently been stocked in the river. A gape
analysis of northern pike might be more informative as to the potential predatory danger of the
species on native fish in the river.
Because catch rates have remained low from 2002 to 2006 and because it is likely a result
of the removal effort, many members of the Program have recommended discontinuing specific
northern pike removal efforts. While we agree that a reduction in effort is certainly warranted,
we are hesitant to completely discontinue this project. Northern pike are known to spawn
immediately after ice-off in early spring (Sigler and Sigler 1996 and references therein). They
are known to have spawned in the middle Green River on one other occasion and their known
concentration areas are ideal spawning habitat for this species (K. Christopherson, pers. comm.).
Northern pike removal is the only project ongoing at this time of year and is highly efficient at
removing other problematic nonnative species such as white sucker. Our recommendation is
therefore to continue a preventative level of early spring nonnative fish removal.
-
29
CONCLUSIONS
We present one conclusion for each of our study goals and a few additional conclusions
meant to help in making recommendations.
• Capture and remove (lethal) adult northern pike from reaches of the middle Green River.
Over the course of the six years, we removed a total of 414 northern pike captured from
this reach of river. Associated catch rates were 0.60 fish/hour electrofishing, 0.16
fish/fyke-net night fyke-netting, and 0.76 fish/hour trammel netting.
• Reduce the abundance of adult northern pike in the middle Green River. After 2003, this
objective was changed to “Maintain low occurrence of adult northern pike in the middle
Green River.” In 2001, crews removed 251 northern pike; this number was reduced to 42
the following year, and remained around 20 individuals through 2006. Catch-per-unit-
effort (CPUE) was also highest for all gear types in 2001 and although CPUE for
electrofishing and fyke-netting varied over the course of the study period, the overall
trend in all gear types was a decrease in catch rates between 2001 and 2006.
• Determine the efficacy of removal efforts. Electrofishing catch rates in the middle Green
are consistently lower than those observed in the Yampa River. Catch rates in 2001 were
comparable to those in the middle Yampa River in 2004; however, subsequent catch rates
have decreased more in the Green River than in the Yampa River and did so immediately
after the initial effort in 2001, pointing to a thus far effective removal effort.
• Identify the means and levels of northern pike control necessary to minimize the threat of
predation/competition on endangered and other native fishes. If one concludes that the
northern pike in the middle Green River are coming predominantly from the Yampa
River, only a minimal (i.e., preventative) amount of effort is required to minimize the
-
30
threat of this predator to native fishes and that this minimal amount of effort will be
required at least until northern pike removal has been successful in the Yampa River.
• The following lines of evidence indicate our removal efforts are successfully limiting the
proliferation of northern pike in the middle Green River: a) declining trend in catch rates
within the Green River, which does not appear to be influenced by extraneous factors
(varying flows, net placement, equipment issues, or personnel turnover); and b) an age
analysis that indicates that northern pike encountered in the middle Green River
periodically immigrate from an upstream source as opposed to recruit from local
reproduction.
• Based on pre-project sampling and the findings of this study, northern pike continue to
concentrate in limited areas: the mouth of Brush Creek, Cliff Creek, Stewart Lake Drain,
Ashley Creek and Sportsman Drain (Table 4). We presume these areas provide the best
habitat for northern pike in the middle Green River. By focusing the majority of our
removal effort in these specific locations in the early spring when fish congregate
(presumably to spawn), we can successfully manage their densities.
• Catch rates tend to be higher with electrofishing and trammel netting; however, fyke nets
can be set and left for many days at a time. Thus, in terms of man-hours, fyke-netting
may be more cost effective in some years (when otters or variable flows are not a
problem).
Deleted:
-
31
RECOMMENDATIONS
• Continue specific early spring removal efforts to effectively manage northern pike prior
to their spawning period. More specifically, be ready to use electrofishing, trammel
netting, and/or fyke netting to capture nonnative fish as soon as the river is navigable
after ice-off.
• Discontinue age analysis using cleithra as northern pike numbers in the middle Green
River are low and have been for a number of years. Given the uncertainties associated
with this technique, if aging is still needed, fin rays or otoliths might be a more reliable
method.
• Discontinue collection and analysis of stomach samples. If more information is desired
on predatory effects of northern pike, initiate a gape analysis.
• Begin the season by using fyke nets to capture northern pike and set nets in both
backwaters and tributary mouths. If problems are encountered with the nets or with either
of these habitats, turn the focus more to electrofishing and trammel netting for northern
pike. Our results show that these methods were the most effective in terms of catch rate
over the course of the study and should therefore not limit our ability to capture northern
pike if fyke-netting becomes problematic.
-
32
LITERATURE CITED
Becker, G.C. 1983. Fishes of Wisconsin. University of Wisconsin Press, Madison.
Beyerle, G.B. and J.E. Williams. 1968. Some observations of food selectivity by northern pike in
aquaria. Transactions of the American Fisheries Society 97(1): 28-31.
Brown, C.J.D. 1971. Fishes of Montana. Agricultural Experiment Station, Montana State
University, Bozeman, Mont.
Bry, C. 1996. Role of vegetation in the life cycle of pike. Pages 45-67 in J.F. Craig. Pike biology
and exploitation. Chapman and Hall, London.
Campana, S.E. 2001. Accuracy, precision, and quality control in age determination, including a
review of the use and abuse of age validation methods. Journal of Fish Biology. 59(2), 197-
242.
Christopherson, K., Native Aquatics Project Leader, Utah Division of Wildlife Resources, 1998
– 2005.
Colorado Division of Wildlife. Unpublished data. Length at age for northern pike captured in
Catamount Reservoir during 2003 and 2004. Age curves for the Yampa River, Stagecoach
Reservoir, and a rangewide curve (Casselman 1996) are presented for reference.
Hawkins, J.A., and T.P. Nesler. 1991. Nonnative fishes of the upper Colorado River basin: an
issue paper. Final Report of the Colorado State University Larval Fish Laboratory to the
Upper Colorado River Endangered Fish Recovery Program, Denver, Colorado.
Hawkins, J., C. Walford, and T. Sorensen. 2005. Northern pike management studies in the
Yampa River, 1999-2002. Final report of Larval Fish Laboratory, Colorado State University,
Fort Collins to Upper Colorado River Endangered Fish Recovery Program, U.S. Fish and
Wildlife Service, Denver, Colorado.
-
33
Lentsch, L.D., R.T. Muth, P.D. Thompson, B.G. Hoskins, and T.A Crowl. 1996. Options for
selective control of nonnative fishes in the upper Colorado River basin. Utah Division of
Wildlife Resources Publication 96-14, Salt Lake City.
Quist, M.C., Z.J. Jackson, M.R. Bower, and W.A. Hubert. 2006. Precision of hard structures
used to estimate age of riverine catostomids and cyprinids in the upper Colorado River basin.
Northern American Journal of Fisheries Management 27: 643-649.
Scott, W.B., and E.J. Crossman. 1973. Freshwater Fishes of Canada. Fisheries Research Board of
Canada, Bulletin 184.
Sharp, D. and D.R. Bernard. 1988. Precision of estimated ages of lake trout from five calcified
structures. Northern American Journal of Fisheries Management 8: 367-372.
Sigler, W.F. and J.W. Sigler. 1996. Fishes of Utah: A natural history. University of Utah Press,
Salt Lake City, 375pp.
Tyus, H.M., and Beard, J.M., 1990. Esox lucius (Esocidae) and Stizostedion vitreum (Percidae)
in the Green River basin, Colorado and Utah. Great Basin Naturalist 50(1): 33-39.
Tyus, H.M., and J.F Saunders, III. 1996. Nonnative fishes in natural ecosystems and a strategic
plan for control of nonnatives in the upper Colorado River basin. Final Report of University
of Colorado Center of Limnology to Upper Colorado River Endangered Fish Recovery
Program, Denver, Colorado.
-
34
Year Electrofishing
Dates
Electrofishing CPUE (Pike/
hour) Fyke Netting
Dates
Fyke Net CPUE
(Pike/fyke net night)
Trammel Netting Dates
Trammel Net CPUE (Pike/
hours) Mean Length all NP (mm)
Length Range all NP (mm)
Total Number
Fish
2001 19 March - 08
May 2.77 20 March -
01 June 0.31 20 March -
29 June 1.03 613 170-871 251
2002 02 April - 10
June 0.60 10 April - 23
May 0.06 16 April - 10
June 0.94 633 295-815 42
2003 31 March - 27
June 0.08 12 March -
20 June 0.02 - - 668 423-984 23
2004 24 March - 14
June 0.20 22 March -
14 June 0.15 26 April - 13
May 0.42 558 371-834 27
2005 11 April - 25
June 0.59 21 March -
16 June 0.06 24 March -
24 June 0.42 638 388-870 32
2006 11 May - 12
May 0.30 27 March -
31 May 0.04 27 March - 30 March 0.00 549 365-790 20
Table 1. Catch rates and total number of northern pike removed during the Northern Pike Removal project (only) from the middle Green River: 2001 – 2006.
Year Electrofishing
Hours Number Pike
Removed Fyke Netting
Nights Number Pike
Removed
Trammel Net/Electrofishing
Hours Number Pike
Removed 2001 19.1 53 420.7 130 37.7 39 2002 28.1 17 165 10 5.3 5 2003 65.5 5 103 2 - - 2004 14.7 3 93 14 2.4 1 2005 8.5 5 227 13 26 11 2006 3.3 1 52 2 2 0
Table 2. Effort expended by gear type during the Northern Pike Removal project (only): 2001-2006.
Electrofishing
Pikeminnow Pop Est Hours
NP Removed
CPUE - CPM Pop
Est
Smallmouth Bass
Removal Hours NP Removed
CPUE - SMB Removal
2001 253 29 0.11 - - - 2002 182.1 10 0.05 - - - 2003 190.4 16 0.08 - - - 2004 - - - 147 9 0.06 2005 - - - 80.4 3 0.04 2006 190.4 16 0.08 152.2 1 0.01
Table 3. Effort and number of northern pike removed during other projects in the middle Green River: 2001-2006.
-
35
Island Park - Rainbow Park (RM334-326)
Split Mountain - Razorback Bar (RM319-311)
Razorback Bar -Thunder Ranch (RM311-303)
Thunder Ranch -Walker Hollow* (RM303-294)
Below Walker Hollow
(
-
36
Figure 1. The Upper Colorado River Basin with the northern pike control reach highlighted. Brush Creek is RKM 490; Cliff Creek is not labeled but is just above Jensen at RKM 487; Stewart Lake is not labeled, but is just above Ashley Creek at RKM 483; and Ashley Creek is RKM 481.
Elkhead Creek
Dinosaur National Monument
Old Charley Wash
Cliff Creek Sportsman’s Drain
Stewart Lake Drain
-
37
Northern Pike Length Frequency - 2001 only
01020304050607080
-
38
Northern Pike Length Frequency - 2004 to 2006
0
2
4
6
-
39
0
0.02
0.04
0.06
0.08
0.1
0.12
2000 2001 2002 2003 2004 2005 2006
Cat
ch-p
er-e
ffort
0
100
200
300
400
500
600
Dis
char
ge (c
ms)
Pikeminnow abundance estimate Smallmouth bass removal Peak Flow Average Flow
Figure 6. Average and peak annual flow (cubic meters per second) as measured at the Green River gauge at Jensen, UT, compared with the northern pike catch-per-unit-effort (number of northern pike/hour ) in the middle Green River from 2001 – 2006 (all other projects).
Northern pike age estimates by year
0
10
20
30
40
50
60
0 1 2 3 4 5 6 7 8 9 10Age
Num
ber o
f fis
h 20012002200320042005
Figure 7. Northern pike age estimates by year.
-
40
Northern pike age class by year
0
20
40
60
80
100
120
140
YOY Juvenile Adult
Num
ber o
f fis
h
20012002200320042005
Figure 8. Northern pike age class by year.
0
50
100
150
200
250
300
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
No.
Nor
ther
n Pi
ke
0
100
200
300
400
500
600
700
800
Dis
char
ge (c
ms)
Northern Pike Peak Flow Average Flow
Figure 9. Average and peak annual flow (cubic meters per second) as measured at the Green River gauge at Jensen, UT, compared with the number of northern pike observed (pre-2001) or removed (2001 and later) in the middle Green River from: 1996 – 2006.